1. Field of the Invention
This invention relates generally to cathode ray tubes, and more particularly is directed to improvements in such cathode ray tubes for reducing the risk of implosion thereof and electrical hazards.
2. Description of the Prior Art
In order to reduce the risk of implosion of the envelope of a cathode ray tube which is usually made of glass, various arrangements are employed for reinforcing the glass envelope, particularly at the face plate thereof. In one of the existing arrangements for reinforcing the glass envelope, a protective panel of a transparent plastic or safety glass covers the face plate and is bonded or fixed to the latter by means of a transparent adhesive resin, such as, a polyester resin, epoxy resin or the like, which is injected into a gap or space between the outer surface of the face plate and the protective panel. However, when such a reinforced or implosion-resistant cathode ray tube is used in a television receiver or the like, there is still the danger or hazard of electrical shock if a viewer contacts the exposed surface of the protective panel at a time when the cathode ray tube is in operation and for a period of time following such operation.
Such electrical shock hazard results from the fact that, during operation of the cathode ray tube, a high voltage of the order of 10 to 30 KV is applied, as an anode voltage, to a conductive coating on the inner surface of the glass envelope. Such high voltage applied at the inside of the envelope causes an electrical charge to be developed on the outer surface of the envelope and such electric charge does not disappear during the period of operation of the cathode ray tube, as well as for a short time immediately after the operation of the cathode ray tube is discontinued. Therefore, if a viewer touches the outer surface of the face plate or of the previously mentioned protective panel which is usually exposed, there is the possibility of such person being subjected to an electrical shock. Since the degree or strength of the electrical shock thus received and its effect on the person being subjected thereof may vary from one individual to the next on the basis of the physical characteristics of such individual, and further may vary in accordance with the temperature and humidity of the environment in which the cathode ray tube is located, it cannot be stated categorically that the electrification of the outer surface of the envelope is dangerous in all cases. However, there is no question that the electric shock that may be received as a result of such electrification is, in any event, a source of considerable discomfort to any adult subjected thereto. Further, the electric shock is dangerous or harmful if received by a young child or by an adjult having a physical condition that makes such person particularly susceptible to damage from electrical shocks. As previously indicated, the electrical shock hazard is not eliminated by the provision of a protective panel disposed in front of the face plate of the tube envelope, as in an implosion-resistant cathode ray tube, as the electrical charge which creates the shock hazard may also be developed on the outer surface of the protective panel.